This invention relates to Identification Friend or Foe (IFF) systems and, more particularly, to adaptive processing providing improved azimuth determination capabilities for IFF interrogator systems adapted for monopulse operation.
The design and implementation of IFF systems are well known. Typically, the frequency of radiated signals relative to the physical and electrical width of IFF system antennas results in a relatively wide radiation pattern. The narrow, horizontally rotated beam of a search radar system provides accurate determination of the azimuth bearing of a target, such as an aircraft. In contrast, the wider beam of a rotated IFF antenna typically does not directly provide adequate target azimuth resolution. As a result, additional processing of received IFF signals is required in order to enhance the capability of determining target azimuth based upon IFF returns from an IFF transponder mounted aboard a target aircraft. While different approaches have been utilized to provide such processing for azimuth determination, tradeoffs have generally been required. As a result, optimum performance has not been available for all operating conditions, which may involve varying levels of received signals in combination with varying levels of interference, such as jamming.
Objects of the present invention are, therefore, to provide new and improved methods of determining target azimuth in an IFF system. More particularly, an object is to provide azimuth determining methods for IFF applications which accomplish one or more of the following operating characteristics and capabilities:
improved azimuth determination; PA1 multi-mode azimuth capability; PA1 monopulse signal processing with adaptive azimuth processing; PA1 automatic selection of azimuth mode responsive to received signal characteristics; PA1 azimuth mode responsive to both relative jamming and noise levels; PA1 simple and reliable adaptive azimuth mode selection; and PA1 avoidance of azimuth accuracy limitations during monopulse operation in the presence of jamming. PA1 (a) receiving reply signals from a target; PA1 (b) deriving monopulse sum and difference signals from the reply signals; PA1 (c) deriving a jamming threshold value based on a level of jamming signals present in the derived monopulse signals; PA1 (d) performing threshold testing utilizing such jamming threshold value and a noise threshold value, with a positive test result indicative that a derived monopulse signal magnitude exceeds both of the jamming and noise threshold values; PA1 (e) on a positive test result in step (d), utilizing a target azimuth value as determined by monopulse azimuth processing using sum and difference signal relative amplitude and phase; and PA1 (f) in the absence of a positive test result in step (d), utilizing a target azimuth value as determined by non-monopulse azimuth processing using signal amplitude relative to antenna pointing azimuth at signal reception.